The invention relates generally to instruments using a laser to produce a guideline and in particular to an improved optical system for projecting a line image.
The use of lasers to project lines on surfaces to guide construction and the like is well known. In one common system, a laser beam having a generally narrow and cylindrical cross-section is directed against an oblique rotating mirror to scan the beam through a plane. When the moving beam strikes a surface, it produces a line image. One such rotating mirror system is described in U.S. Pat. No. 4,221,483. The alignment of the mirror in this system is critical to producing a straight line and the required precision and the need for a rotating mechanism can make such systems relatively expensive and delicate.
The moving mirror may be eliminated through the use of a stationary conical mirror in which the laser beam is directed along the axis of the cone toward its apex and reflected over 360 degrees therefrom. While such conical optic systems avoid some of the costly mechanism of a rotating mirror system, conical mirrors of high accuracy are difficult and expensive to produce.
For this reason it is known to replace the conical mirror with an anamorphic diverging lens. The lens serves to expand the laser beam in one dimension only thereby producing a plane of light whose image is also a line. While such systems using a diverging lens are potentially more robust and less expensive than mirror systems, the amount of divergence achievable is far less than that obtained with the mirrors. U.S. Pat. No. 5,461,790 describing such a lens based system notes the dispersion of the beam is limited to 30 to 60 degrees, far less than the 360 degrees often attainable with a rotating mirror system. Further, although the diverging lens is more robust and less expensive than the mirror systems, the fabrication and alignment of anamorphic lenses can be difficult and expensive.